This project examines at the subcellular level the enzymatic processes involved in the regulation of intracellular high energy phosphate. It is specifically focused upon the coordination of mitochondrial oxidative phosphorylation to the bioenergetic demands of excitation-contraction, and the possible participation of the creatine-phosphocreatine system in these critical events. A detailed investigation is being conducted on the properties of the three major heart isoenzymes of creatine kinase. Specific emphasis is placed upon their special catalytic function, regulation, subunit structure, amino-acid composition, and substrate binding properties. An original contribution of this study has been the exploration of energy-channeling directed by the specific microcompartmentation of the creatine kinase isoenzymes. Information being obtained from this research contributes unique knowledge in two areas of cardiac bioenergetics: 1) the molecular mechanisms by which high-energy phosphate transfer is affected and controlled at the enzymatic level; and 2) how energy is directed from the mitochondrion to specific sites of utilization, i.e., the myofibrils, the sarcoplasmic reticulum, and the sarcolemmal membrane.